Field of the Invention
The present invention relates to an R-T-B rare earth sintered magnet, an alloy for an R-T-B rare earth sintered magnet, and a method of manufacturing the alloy, and particularly, to an R-T-B rare earth sintered magnet having excellent magnetic properties.
Priority is claimed on Japanese Patent Application No. 2013-000445, filed on Jan. 7, 2013 and Japanese Patent Application No. 2013-256492, filed on Dec. 11, 2013, the contents of which are incorporated herein by reference.
Description of Related Art
Hitherto, R-T-B rare earth sintered magnets (hereinafter, may be referred to as “R-T-B magnets”) have been used in motors such as voice coil motors of hard disk drives and motors for engines of hybrid vehicles and electric vehicles.
R-T-B magnets are obtained by molding an R-T-B alloy powder containing Nd, Fe, and B as main components and by sintering the resulting molded product. In general, in R-T-B alloys, R is Nd, part of which is substituted by other rare earth elements such as Pr, Dy, and Tb. T is Fe, part of which is substituted by other transition metals such as Co and Ni. B is boron, part of which can be substituted by C or N.
Normal R-T-B magnets have a structure constituted mainly of a main phase consisting of R2T14B and an R-rich phase which is present at the grain boundaries of the main phase and has a higher Nd concentration than the main phase. The R-rich phase is also referred to as a grain boundary phase.
In general, regarding the composition of R-T-B magnets, the ratios of Nd, Fe, and B are adjusted to be as close to R2T14B as possible, in order to increase the ratio of the main phases in the structure of an R-T-B magnet (for example, see Permanent Magnet-Materials Science and Application—(Masato Sagawa, Nov. 30, 2008, second print of the first edition, pgs. 256 to 261)).
In addition, R-T-B alloys may include an R2T17 phase. The R2T17 phase is known as a cause of a reduction in coercivity and squareness of R-T-B magnets (for example, see Japanese Unexamined Patent Application, First Publication No. 2007-119882). Therefore, hitherto, an R2T17 phase has been eliminated during the course of sintering in order to manufacture an R-T-B magnet when the R2T17 phase is present in an R-T-B alloy.
In addition, since R-T-B magnets which are used in motors for vehicles are exposed to high temperatures in the motors, high coercivity (Hcj) is required.
There is a technology used to replace Nd with Dy for R of an R-T-B alloy as a technology used to improve the coercivity of the R-T-B magnet. However, Dy is unevenly distributed and its output is also limited. Accordingly, the supply of Dy is unstable. Therefore, technologies, which improve the coercivity of an R-T-B magnet without increasing the amount of Dy contained in an R-T-B alloy, are known.
There is a technology which adds a metal element such as Al, Si, Ga, and Sn in order to improve the coercivity (Hcj) of an R-T-B magnet (for example, see Japanese Unexamined Patent Application, First Publication No. 2009-231391). In addition, as described in Japanese Unexamined Patent Application, First Publication No. 2009-231391, Al and Si are known to be mixed as inevitable impurities into the R-T-B magnet.
In addition, All about Neodymium Magnet-Let's Protect Earth with Rare Earth-(Masato Sagawa, Apr. 30, 2011, first print of the first edition, pgs. 104 to 105) states that it is desirable to cause crystal grains of a magnet to have a shape extended in a direction of an axis of easy crystal magnetization, in order to minimize the influence of magnetostatic interaction when adjacent grains are subjected to magnetization reversal.